The invention relates to the field of nuclear medicine, notably to a scintillation camera which comprises a scintillation crystal which serves to convert each photon received into a scintillation, a light guide for coupling said crystal to the entrance window of an array of p photodetectors which serve to convert each scintillation into a current, p acquisition channels which receive the output signals of said photodetectors and which supply p characteristic electric signals which relate notably to the intensity of the scintillation and to the distance between the respective scintillation and each of the photodetectors, and a processor which serves to supply the coordinates x.sub.j and y.sub.j of a scintillation j and its associated energy E.sub.j.
For the determination of the image of the radioactive distribution inside an organ, medical diagnostics utilizes inter alia the scintigraphy principle. This principle is based on the introduction of a radioactive element into the organism of a patient, said element attaching itself more or less to given organs, depending on whether these organs are healthy or not. The measurement of the intensity of the gamma radiation emitted provides an indication of the distribution of the radioactive element in the organism and hence forms a diagnostic aid. A measurement of this kind is performed by means of a scintillation camera.
In conventional scintillation cameras, for example Anger type cameras (the physician Anger was the first one to propose a scintillation camera whose principles are described in U.S. Pat. No. 3,011,057), the gamma rays which are representative of the radioactive distribution in the environment examined enter a scintillation crystal after having passed through a collimator. The scintillations produced in the crystal are subsequently detected by a series of photomultiplier tubes (for example, 37) after having passed through a light guide which optically couples the crystal to the tubes. The tubes are distributed in front of the optical block (crystal +light guide) so as to cover substantially the entire surfaces thereof and convert the light energy of each scintillation occurring into a measurable electric signal.
To the output of each photomultiplier tube there is connected an analog acquisition channel which successively provides amplification, integration and shaping of the signals supplied by the tube. The output signals S.sub.ij of the array of acquisition channels are applied to a processor which supplies, by estimation, the coordinates x.sub.j and y.sub.j of a scintillation j and its energy E.sub.j (the index i designates the relevant acquisition channel). The processor may comprise several types of calculation devices, but essentially two thereof are used in practice, i.e. a device for calculating the center of gravity on an arithmetical basis (referred to hereinafter as an arithmetical calculation device) and a device for calculating the center of gravity of a logarithmic basis (referred to hereinafter as a logarithmic calculation device).
In an arithmetical calculation device the quantities x.sub.j, y.sub.j, E.sub.j are given by the expressions: ##EQU1##
In these expressions: ##EQU2##
The coefficients G.sub.i, K.sub.i, H.sub.i, J.sub.i are weighting factors relating to the position of the axis of each of the p photomultiplier tubes.
In a logarithmic calculation device, the quantities x.sub.j, y.sub.j, E.sub.j are given by the expressions: ##EQU3## The weighting factors are again related to the position of the axis of each of the p photomultiplier tubes.
Regardless of the arithmetic used, a modified coordinate calculation has been proposed in order to improve the intrinsic spatial resolution of the camera. This modification, presently used in the majority of scintillation cameras developed for nuclear medicine and described, for example in U.S. Pat. No. 3,732,419, consists of the introduction of a threshold s.sub.o in the calculation of the composite signals X.sub.j, Y.sub.j, Z.sub.j or X.sup.+.sub.j, X.sup.-.sub.j, Y.sup.+.sub.j, Y.sup.-.sub.j.
More precisely, taking by way of example an arithmetical calculation device, the composite signals X.sub.j, Y.sub.j are now given by the expressions: ##EQU4## with, moreover, the convention S.sub.ij -s.sub.o =0 if S.sub.ij is smaller than s.sub.o. The relations (1), (2), (3), providing x.sub.j, y.sub.j, E.sub.j, and the relation (6) remain the same.
The introduction of such a threshold is realized by inserting a threshold circuit in each of the p analog acquisition channels. The p channels thus have two outputs: one for the signals S.sub.ij -s.sub.o which enable the calculation of X.sub.j and Y.sub.j, and one of the signals S.sub.ij used for the calculation of Z.sub.j and E.sub.j. The value s.sub.o of the threshold is fixed and has been adjusted once and for all during manufacture in order to obtain the best spatial resolution for a given energy, for example 140 keV.
However, because their adjustment is independent of the real energy of the scintillations, the performance of the cameras deteriorates (notably linear distortions and resolution losses) for scintillation energies which deviate excessively from those occurring upon adjustment during manufacture. This is particularly the case when multi-isotope examinations are performed.
According to U.S. Pat. No. 4,475,042, this drawback is eliminated by utilizing a scintillation camera having a dynamic threshold which has an amplitude which is a function of the time and the energy of the scintillation being processed. This threshold (inserted, like the fixed threshold mentioned above, in the p analog acquisition channels at the output of the preamplifiers associated with the photomultiplier tubes) is formed by the sum of two signals. The first of these two signals is a fraction of the energy signal resulting from the weighted summing of the signals supplied by all preamplifiers, without threshold effect. The second signal is a fraction of the integrated energy signal which corresponds to the integrated value of the first of the two signals. If desirable, a fixed component may also be superposed on these two signals.
The threshold signal thus formed presents, as a function of time, statistical fluctuations, that is to say fluctuations linked to the number of photons relative to each of the scintillations involved in the observation. The mean value of these fluctuations is linked to the energy of the scintillation being processed. This threshold is subtracted from each of the signals supplied on the output of the preamplifiers.
The improvement offered by U.S. Pat. No. 4,475,042 enables the image quality to be maintained when a plurality of isotopes are simultaneously used.
However, in practice this solution still has considerable drawbacks:
(a) the signals forming the dynamic threshold are obtained only after deformation and delays determined by the transfer function of the circuits traversed;
(b) the dynamic threshold is a signal which fluctuates at random as a function of time and also varies from one scintillation to another, which different fluctuations and statistical variations of the relative amplitude and phase for the signals supplied by the preamplifiers and for the dynamic threshold lead to the phenomenon that the signals supplies often are, instead of being a signal zero or a useful signal, noise which degrades the spatial resolution again;
(c) the dynamic threshold is no longer suitable for high counting rates when at least a partial pile-up of the scintillations occurs because the mean value of the dynamic threshold is in that case no longer linked exclusively to the energy of the single scintillation being processed, so that it is incorrect;
(d) given tubes of the set of photomultiplier tubes involved with a given scintillation are situated in an intermediate position at such a distance from the respective scintillation that the maximum amplitude of the pulses supplied by the preamplifiers is only two or three times higher than the mean amplitude of the dynamic threshold. For the acquisition channels associated with these tubes, these output pulses are incomplete because the dynamic threshold elminates exactly the lower terminal portion of these pulses.
It is an object of the invention to propose a scintillation camera in which these drawbacks are mitigated, notably, because the above embodiments comprising a dynamic threshold are not suitable for high counting rates.
To this end, the scintillation camera in accordance with the invention is characterized in that:
(A) said p acquisition channels realize the amplification, filtering and sampling of said output signals of the photodetectors, followed by the A/D conversion of the samples obtained and their summing, and apply p digital signals to the input of the processor;
(B) the processor itself comprises:
(a) a bus for transferring said p digital signals: PA1 (b) a digital summing stage which itself comprises: PA1 (c) a scintillation processing stage which includes unpiling calculation circuits and two dividers and which supplies, on the basis of the signals X.sub.mj, Y.sub.mj, E.sub.mj, E.sub.mj, the three coordinate and energy signals x.sub.j, y.sub.j, E.sub.j relating to the scintillation j;
a buffer memory which receives the p output signals M.sub.ij of the transfer bus; PA2 a threshold subtraction circuit, a first input of which receives said p signals, delayed by the buffer memory, while a second input thereof receives a dependent threshold value m.sub.oj, said subtraction circuit supplying p signals M.sub.ij -m.sub.oj which assume the value 0 if M.sub.ij is smaller than m.sub.oj ; PA2 four digital weighted sum forming devices, the first two of which receive the p signals M.sub.ij -m.sub.oj supplied by the threshold subtraction circuit, while the other two receive directly the p output signals M.sub.ij of the transfer bus; PA2 a threshold calculation device which receives the output signal of the fourth digital weighted sum forming device in order to evaluate the dependent threshold m.sub.oj according to a proportionality relation with the value E.sub.mj associated with each scintillation j, said digital summing stage supplying the following signals X.sub.m,j Y.sub.m,j, Z.sub.m,j, E.sub.m,j (K.sub.i, H.sub.i, J.sub.i, G.sub.i being weighting coefficients): ##EQU5## either directly on the output of the first two digital weighted sum forming devices for the first two of these signals or via two time realignment circuits, connected to the output of the other two devices, for the last two of these signals;
(C) a detection, sequencing and storage stage which receives a signal which corresponds to the sum of the p output signals of the photodetectors is provided in order to supply on the one hand the various clock signals for synchronising the elements of the p acquisition channels and the elements of the processor, and on the other hand the correction coefficients for the scintillation processing stage.
In two alternative embodiments of the proposed camera the processor can utilize only one of the two quantities Z or E for calculating the coordinates, taking into account the later correction possibilities offered by contemporary scintillation cameras. In that case the processor comprises only three digital weighted sum forming devices, the first two of which receive the p signals M.sub.ij -m.sub.oj supplied by the threshold subtraction circuit, while the third device directly receives the p output signals M.sub.ij of the transfer bus. The threshold processing device thus receives the output signal of said last digital weighted sum forming device for the evaluation of the dependent threshold m.sub.oj according to a proportionality relation with the value E.sub.mj associated with each scintillation j.
In the first of these two alternative embodiments, the digital summing stage supplied the following signals X.sub.m,j, Y.sub.m,j, Z.sub.m,j (K.sub.i, H.sub.i, J.sub.i being the weighting coefficients): ##EQU6## either directly on the output of the first and the second digital weighted sum forming device for the first two of these signals or via a time realignment circuit, connected to the output of the third device, for the last one of these signals.
In the second alternative embodiment, the digital summing stage supplies the following signals X.sub.m,j, Y.sub.m,j, E.sub.m,j (K.sub.i, H.sub.i, G.sub.i being the weighting coefficients): ##EQU7##
These signals are also applied either directly on the output of the first and the second digital weighted sum forming device for the first two of these signals, or via a time realignment circuit, connected to the output of the last device, for the last one of these signals.
As a result of the use of the threshold dependent on the energy of the scintillations as well as on the pile-up effect of these scintillations, in both camera embodiments an optimum spatial resolution can be obtained, regardless of the counting rate, throughout the entire energy spectrum of the radiation used. The scintillation camera thus obtained is faster and has a superior performance in comparison with the previous embodiments and, moreover, is particularly suitable for multi-isotope examinations.